Titanium (Ti) has a melting point of 1,600° C. or higher, which is far higher than those of aluminum (Al) and magnesium (Mg), which are likewise classified as light metals. Furthermore, at 885° C., which is a β-transus (transformation point), titanium undergoes an allotropic transformation in which the crystal structure changes from the close-packed cubic system (α phase) to the body-centered cubic system (β phase). These properties are utilized to develop Ti alloys.
Many heat-resistant Ti alloys contain Al, which is an element for stabilizing the α phase, which has excellent high-temperature strength, and the mechanism of solid-solution strengthening by Sn or Zr is utilized therein. Representative alloys include a Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6-2-4-2S) alloy, which is used as members for airplane engines. This alloy is regarded as combining high mechanical strength and creep resistance even at temperatures around 750 K.
For example, Patent Document 1 discloses, for Ti-6-2-4-2S alloy, a method of adjusting the grain sizes of the metallographic structure which are capable of affecting the mechanical strength by changing conditions for heat treatments and forging. Specifically, the patent document first discloses that a work is processed in accordance with specification of AMS4976, namely, a work is hot-worked at a temperature which is in an (α+β)-two-phase temperature region and is close to a β-transus, and is then heat-treated at a temperature lower than the β-transus by several tens of degrees centigrade to perform an aging treatment, thereby obtaining a heat-resistant Ti alloy having a composite structure including a β phase in which an acicular α phase and an equiaxed α phase have been formed (see the description in paragraph [0133] and FIG. 11(a)). In addition, the patent document discloses a method in which an alloy having a β-transus of, for example, 996° C. is subjected to β annealing at a temperature higher than the β-transus and is then subjected to hot working at a temperature that is in an (α+β)-two-phase temperature region and is lower than the β-transus by 56-388° C. and at a given strain rate, thereby enabling the acicular α phase and the equiaxed α phase to be formed more thinly (see the description in paragraph [0134] and FIG. 11(b)).
Meanwhile, Patent Document 2 discloses an improved material of a Ti-6-2-4-2S alloy, and discloses that the amount of an equiaxed α phase obtained by hot forming is adjusted by performing a solution heat treatment, thereby enabling the improved material to combine fatigue strength and creep strength at high temperatures. Specifically, a bulk alloy having a given composition is held in a β-single-phase temperature region, rapidly cooled to 700° C. or lower by air cooling or at a rate not lower than that in air cooling, and then gradually cooled by air cooling or at a rate not higher than that in air cooling. Subsequently, the alloy is hot-formed in an (α+β)-two-phase temperature region and thereafter subjected to a solution heat treatment and then to an aging heat treatment. In particular, the patent document discloses that the hot forming is performed so as to result in a forming ratio of 3 or higher to obtain an equiaxed α phase in a sufficient amount. The patent document discloses that, in general, the creep strength can be enhanced by adjusting the holding temperature in the solution heat treatment to a temperature in a β-single-phase temperature region to reduce the amount of the equiaxed α phase, while the fatigue strength can be enhanced by adjusting the holding temperature to a temperature in an (α+β)-two-phase temperature region to increase the amount of the equiaxed α phase.
Patent Document 1: JP-T-2016-503126
Patent Document 2: JP-A-10-195563